I. This study will measure some of the determinants of three important mechanisms of the sodium transport system in human red blood cells. 1. Sodium/lithium countertransport. 2. Sodium-potassium cotransport. 3. Circulating inhibitor of sodium-potassium ATPase. II. The population will be 75 men and 75 women with blood pressure in the high normal range who are participating in a study of the effects of potassium supplementation and changes in sodium/potassium ratio on blood pressure. III. The following types of studies will be done. 1. The cross-sectional relationship at baseline between the three mechanisms of transport mentioned in I above, age, sex, blood pressure, family history of high blood pressure, urinary excretion of sodium and potassium, obesity, education, income, cigarette smoking, alcohol intake, serum Na, K, and creatinine, and urinary Na+ and creatinine. 2. The changes in the three mechanisms of the transport system after one year in relationship to dietary changes in potassium and sodium compared to a control diet. 3. The potential for determining which aspects of red blood cell sodium transport will identify a group of individuals whose blood pressure will change in response to an increase in potassium in their diet over one year.